Despite what instructions may say, just because the bottles may feel hard doesn't mean the co2 is actually locked into the beer yet. So it's not fully carbed.
There is one simple reason...it's too soon.
The
3 weeks at 70 degrees, that we recommend is the
minimum time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer. Lower temperatures take longer.
Stouts and porters have taken me between 6 and 8 weeks to carb up..
I have a 1.090 Belgian strong that took three months to carb up.
Temp and gravity are the two factors that contribute to the time it takes to carb beer. But if a beer's not ready yet, or seems low carbed, and you added the right amount of sugar to it, then it's not stalled,
it's just not time yet.
Everything you need to know about carbing and conditioning, can be found here
Of Patience and Bottle Conditioning. With emphasis on the word,
"patience." 
Read the above blog, and come back to the beer in a couple more weeks.
If a beer isn't carbed by "x number of weeks" you just have to give them ore time. If you added your sugar, then the beer will carb up eventually, it's really a foolroof process. All beers will carb up eventually. A lot of new brewers think they have to "troubleshoot" a bottling issue, when there really is none, the beer knows how to carb itself. In fact if you run beersmiths carbing calculator, some lower grav beers don't even require additional sugar to reach their minimum level of carbonation. Just time.
Also, you are using 1 liter bottles. The above time frame is for
12 ounce bottles, NOT 1 liter ones.
A larger volume sized bottle usually needs more time to carb AND condition. I have some pints, 22 oz bombers and other sizes that I often use, but since I enter contests I usually also do a sixer or two of standard 12 ouncers for entering. And inevitably the 12 ouncers are done at least a week faster than the larger bottles....some times two weeks ahead of time...
Also the rule of thumb is 3 weeks at 70 degrees for a normal grav 12 ounce bottle....to carb and condition....It takes longer for the yeasties to convert the larger volume in the bigger bottles to enough co2 in the headspace to be reabsorbed back into the solution...A ration I don't know how much...
Big Kahuna gives a good explanation here...
Simple. It's the ration of contact area just like in a keg. The c02 will need to pressurize the head space (Which takes LESS TIME) in a bigger bottle (More Yeast and sugar, roughly the same head space) but then it has to force that c02 into solution through the same contact area...thus it takes longer.
Just forget about them for a couple more weeks, and you will find that you should have plenty of carbonation and plenty of head.
